OPINION input requested

Ref: Discussion of SuperHots,
I expressed the following OPINION If I'm wrong, don't pull any punches, just tell me where I went wrong.
bob
In my OPINION, whether the centerline of the crankshaft is directly ON the published thrust line,, or a bit above or below that line should have relatively little effect on how the plane operates. the firewall on this plane is relatively tiny, so to get more than around 1/2 inch away from the published number is going to be a bit difficult.
If my OPINION is wrong, perhaps some of the folks who understand it better could jump in and tell me where my reasoning is at fault.
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Agreed 100%.

I'll take your word for that.
Evidence for why I think the centerline of the crankshaft doesn't have to be on the centerline of the fuselage: any number of amphibians and gliders that have the thrustline (which follows the engine wherever it goes) well above the centerline and center of mass of the aircraft.
The engines are mounted on pods or on the vertical stab.
So airplanes can be made to fly with that kind of arrangment.
Having said that, it also seems reasonable to think that as you move the thrustline above the centerline and center of mass of the aircraft that stuff will happen to the flight characteristics. Whether we would notice the change depends a lot on the quality of the pilots and the design of the aircraft.
I'll bet that a 1/2" change up or down on the firewall could be trimmed out without any difficulty and that after trimming no one could tell the difference. There **has** to be a calculable difference in the thrust vectors because of the difference in location, but that does not mean that it would be noticeable from the ground.
                Marty
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Bob Cowell wrote:

I suspect it won't make a heck of a lot of difference. If you're flying for sport, it probably won't be significant. If you're looking for all-out precision aerobatic performance, then that 'not a heck of a lot of difference' may be more than enough to worry about, at least once.
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Tim Wescott
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Bob Cowell wrote:

Depends on what you mean by 'relatively little'.
Its certainly enough to make e.g. a pattern plane completely untrimmable for precision flight.
Its barely going to affect an average sort of sport model, which is like that (untrimmable for precision flight) out of the box anyway..
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For the average run of the mill bird, you are correct. For a precision aircraft, that is a disaster. The issue is thrust vector changes when the throttle changes. Think about the old flat bottomed wing trainers and the down thrust built in and you will realize the impact of that 'off center line' installation.
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It's like this:     if the thrustline is a little higher than the original design, the plane will tend to nose over (down) a little more than before, all other things being unchanged, if the thrustline is lower than before, the plane will tend to nose up. These effects may be infinitesimal and can be easily trimmed out, of course. A small change in layout will beget a small change in flight characteristic.     Chances are the model designer just drew the plans the way that looked good to him, or as an evolutionary development of a previous design and never did any engineering or calculations at all to check the effects of the given thrustline, especially if it's a non-scale design. It was probably based on his experience and knowledge, so it's possible what you're doing could actually improve the way the airplane flys!     It's incredible the misconceptions that some of these guys believe, and even hold dear to their hearts. The "straight-back" airfoil being no good for models myth comes to mind.     -Uncle Pauly
Bob Cowell wrote:

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